System and Method for Recovery of Phosphate from Wastewater as Granular Natural Struvite

This invention claims the benefit of U.S. Provisional Patent Application No. 63/642,285, filed May 3, 2024, the entirety of which is hereby incorporated herein by this reference.

This invention was made with government support under grant number 2241633, awarded by the National Science Foundation. The government has certain rights in the invention.

The present invention generally relates to recovering minerals from wastewater. More particularly, the present invention relates to a system and method for recovering phosphate and ammonium from wastewater and converting into granular products for use in agricultural production and commercial applications.

Recovery of struvite (MgNHPO·6HO) from phosphate-rich wastewater such as source-separated human urine and anaerobic digestate has gained growing interest in research and commercial applications. It aligns well with the principles of sustainable wastewater treatment and circular economy. Phosphate-rich wastewater can include urine, digestate, liquid animal manure, and some other industrial wastewater stream. Struvite forms in equal mole numbers of ammonium (NH), magnesium ions (Mg), and inorganic phosphate (PO, HPO, and HPO) and is favorable at pH 8.0-9.5. Nevertheless, phosphate-rich wastewaters often have low pH values and a lower magnesium concentration relative to ammonium and phosphate, demanding the use of external alkali and magnesium sources for struvite precipitation as shown in:

Mg+NH+HPO+6HO→MgNHPO·6HO+H

Commonly, magnesium is supplemented with soluble salts such as MgCland MgSOand NaOH is added to increase pH, which often account for up to 75% of the operational cost for extant struvite recovery. Seawater and desalination bittern are low-cost alternative sources of magnesium ions, though they are locally available only and increase wastewater ionic strength and concentrations of interfering ions.

Cheaper Mg(OH)and MgO can be used to supplement Mgand raise pH for phosphate-rich wastewater, but the optimum dosages for pH increase and Mgsupplementation are often different, and their solubilities are relatively low. There have been difficulties to find cost-effective sources of magnesium and alkali for struvite recovery from the near-neutral wastewater.

Additionally, magnesium and alkali should be supplemented from a natural or organic source so that the recovered struvite has the potential to be used for certified organic production. Organic production is characterized by prohibiting the use of chemically synthesized fertilizers. Organic producers are certified according to U.S. Department of Agriculture National Organic Program standards through approved certification bodies. For soil fertilization, organic systems rely on the inputs of fertilizers obtained from crushed phosphate rocks and organic sources such as animal manure, compost, poultry droppings, meat/bone/blood meal, biofertilizer, and biosolids.

Commonly, all such organic and natural mineral fertilizers have limited availability, low nutrient rating, and inconsistent nutrient content, and rely on soil microbes for their nutrient mineralization, leading to crop yields from organic farming on average 25% lower than conventional farming. Therefore, there is a high demand for high-grade natural fertilizers to increase the productivity of organic production systems. Natural fertilizer is a substance composed only of natural organic and/or natural inorganic fertilizer materials and natural fillers. When struvite crystals are recovered from wastewater without use of synthetic chemicals, the recovered crystals can be considered natural inorganic fertilizers and are expected to be accepted for certified organic production.

Recovered struvite and precipitated phosphate salts have been approved for use in organic production by their inclusion in annex II of Regulation 2021/1165 (European Commission, 2023). The Permitted Substance List of Canadian Organic Production Standard (Canadian General Standards Board, 2021) permits all sources of magnesium in the manufacturing process of struvite with livestock manure as well as plant and plant byproducts. According to the U.S. National Organic Program standards, however, struvite recovered from wastewater is considered a synthetic fertilizer, thus being prohibited in certified organic production (USEPA, 2023).

Most organic regulations do not allow chemical processing or use of synthetic compounds in the manufacturing processes, but physical and mechanical processing are allowed. Inputs to organic lands should be natural substances or naturally acquired materials. Therefore, natural minerals after physical or mechanical processing offer a prospect to recover struvite from wastewater for organic production.

Minerals such as brucite (Mg(OH)·6HO), serpentine (MgSiO(OH)), and magnesite (MgCO) have been explored for magnesium supplementation and pH elevation. However, acid dissolution required for brucite is not suitable for producing struvite for organic production. The recovery efficiency by addition of milled serpentine has been shown to decrease with increasing POconcentration greater than 7 mM since Si released from serpentine acted as seeds for secondary struvite crystallization, making it unsuitable for commercial-scale struvite recovery from wastewater with high phosphate concentrations.

Magnesite has magnesium embedded in its crystal lattice, rendering a low solubility in water. However, calcination of magnesite converts it to magnesium oxide (equation below) which is more soluble than ground magnesite. The dissolution of calcined magnesite in water generates hydroxyl ions which increase water pH. Using calcined magnesite for struvite recovery costs less than bittern and chemicals including MgO, MgCl, and MgSO.

Compared with milled magnesite and milled serpentine MgSiO(OH), a smaller dosage of calcined magnesite is needed due to its high magnesium content, thus minimizing the residuals left in the wastewater.

Some have evaluated the effects of calcination temperature and duration on the dissolution of calcined magnesite in rare-earth processing wastewater, synthetic phosphate-depleted human urine, acidic electroplating wastewater, and liquid digestate, respectively. However, the calcination process has not been optimized for dissolution of calcined magnesite in real alkaline phosphate-rich wastewater like hydrolyzed urine and air-stripped anaerobic digestate. The dissolution kinetics and capacity of calcined magnesite are determined by reactivity and the concentration gradient around the particles of calcined magnesite in a solvent. The reactivity of calcined magnesite is affected by calcination temperature and residence time.

Mineral-derived water-soluble magnesium salt, for example, the OMRI-listed Magriculture® Epsom salt (MgSO·7HO crystals), can be directly added for magnesium supplementation. Nevertheless, chemical-free pH elevation is still required for most wastewater streams to precipitate struvite for organic production. Moreover, addition of Epsom salt alone to supplement magnesium increases ionic strength and decreases the activity of struvite ions, which may decrease the effectiveness of struvite formation.

When only calcined magnesite is dosed to reach a Mg:POratio greater than 1, however, pH may fall outside the optimum range for struvite formation. Vice versa, Mg:POratio may not be appropriate for efficient phosphate removal when calcined magnesite is dosed to reach the optimum pH range.

Commercial fertilizers are mostly marketed in granulated form. The existing commercial-scale struvite recovery systems produce granular struvite by days of crystal growth inside a reactor. The studies that supplement magnesium with mineral products have not produced a marketable granular fertilizer, neither for conventional nor organic farming. The present invention is to induce struvite precipitation and crystallization with mineral products, harvest fine struvite crystals upon a short crystal residence time, and mechanically turn struvite powder into fertilizer granules with biodegradable organic binding agents.

The present invention therefore provides an advantage in extracting struvite from wastewater for use in agricultural and commercial applications. The present invention is industrially applicable as it produces struvite that can be used as fertilizer, even in organic settings.

Briefly described, the present system and method utilize the combined use of calcined magnesite and a water-soluble magnesium mineral as an inclusive approach to recovering struvite from various wastewaters for organic production. The invention uses the efficacy of struvite recovery from alkaline hydrolyzed urine as well as from near neutral and slightly alkaline liquor of sludge digestate and animal manure digestate with the addition of both calcined magnesite and Epsom salt. Thus, the invention provides a method of using an organic-certified water-soluble mineral salt alone or a combination of calcined magnesite and the mineral salt to produce struvite. The calcination process is optimized for light-burnt magnesite for struvite recovery from phosphate-rich wastewater and can be upscaled to extract phosphate from a continuous waste stream.

The present invention integrates simple pretreatment by air stripping, mineral-based struvite crystallization, and bio-based granulation in tandem for production of granular natural struvite fertilizers.

In one embodiment, the invention provides a system for recovering phosphate from wastewater with a liquid container having an interior thereof and an outlet therefrom, and the interior configured to receive wastewater in at least a semi-liquid form, where the wastewater containing at least phosphate. There is a filter in the outlet of the liquid container that will remove the struvite. There is a calcined magnesite source and a water-soluble magnesium mineral source to provide these to the wastewater.

There is a control system configured to selectively add the calcined magnesite source to wastewater, then selectively add the water-soluble magnesium mineral source to the wastewater until a predetermined molar ratio of magnesium to phosphate thereby creating a mixture of the calcined magnesite source, water-soluble magnesium mineral source, and wastewater. This process ultimately forms struvite in the mixture and the struvite is collectable in the outlet of the liquid container and can be externally pressed into granules to be used as a granular natural fertilizer.

The system can also include an air source connected to the interior of an air-lift crystallizer and operably connected to the control system such that the control system further selectively controlling a predetermined pressure and flow rate of the air supplied into the crystallizer.

Additionally, the system can further contain one mixer to make the calcined magnesite source into slurry prior to dosing it into the liquid container. The calcined magnesite source, water-soluble magnesium mineral source, and wastewater can all be held inside the container, or can be continuously flowed through the container as a mixture with the filter further configured to continuously remove struvite from the flowing mixture.

Following the liquid container that produces struvite crystals, the system can contain a post-crystallization granulator to quickly convert the crystals into granular struvite fertilizers. At the ratio of 10 grams of struvite crystals to 0.8 grams of organic binder to 5.4 grams of water, the purity of the struvite granules exceed the Association of American Plant Food Control Officials' benchmark for a marketable compound fertilizer. Furthermore, the struvite granules have a great potential to be listed as a crop fertilizer for certified organic production when National Organic Program allowed organic polymers are used as binding agents.

In an embodiment, the invention can also provide a method for recovering phosphate from wastewater by the steps of streaming wastewater through a liquid container having an interior thereof and an outlet therefrom, with the wastewater in at least a semi-liquid form and containing at least phosphate, then adding a calcined magnesite source to the wastewater adding a water-soluble magnesium mineral source to the wastewater until a predetermined molar ratio of magnesium to phosphate thereby creating a mixture of the calcined magnesite source, water-soluble magnesium mineral source. The method continues by forming struvite in the mixture and filtering the struvite from the mixture.

Simple air stripping of COand sedimentation as quick pretreatment is applied to raise pH and subsequently remove Mg-competing cations so as to increase downstream struvite product purity.

The present invention therefore provides an advantage in obtaining phosphates from wastewater that would otherwise be pollutive and require special disposal. The struvite produced herein can be used as natural fertilizer for traditional farming and organic production. The present invention is also industrially applicable as it can produce commercially viable organic-certifiable fertilizer from a wastewater stream. Other objects, advantages, and features will become apparent to one of skill in the art from review of the present application.

With reference to the figures in which like numerals represent like elements throughout the several views,is a systemfor recovering phosphate from a wastewater sourcewith a liquid container, shown here as a crystallizer, having an interiorthereof and an outlettherefrom, and the interiorconfigured to receive wastewater in at least a semi-liquid form, and the wastewater contains at least phosphate. There is a filter(shown here as a sieve) in outletof the liquid containerthat will remove struvite that is produced in the manner more fully described below. There is a calcined magnesite sourceand a water-soluble magnesium mineral sourceto provide these to the wastewater from the wastewater source. The containercan be an air-lift crystallizer, a simple container, a liquid pass such as an elongate tube, or other like structure sufficient to execute the functionalities described herein.

There is a control systemconfigured to selectively add the calcined magnesite sourceto wastewater in the interiorof the container, then selectively add the water-soluble magnesium mineral sourceto the wastewater until a predetermined molar ratio of magnesium to phosphate thereby creating a mixtureof the calcined magnesite source, water-soluble magnesium mineral source, and wastewater. This process ultimately forms struvite in the mixture and the struvite is collectable in the filterand can be used as an organic-certifiable natural fertilizer.

The crystallizer has a riser where compressed air push water-crystal fluid up through it. Rising air bubbles burst at the top of the riser while water-crystal fluid overflows and circulates through the downcomer between the riser and the crystallizer wall. Recirculation of water-crystal-air mixture facilitates crystal growth.

The systemcan include a heating elementinteracting with the milled magnesite for calcination. The systemcan also further include water-soluble magnesium mineral sourcesuch that the control systemfurther configured to selectively add the mineral salt to the mixture. Further, the calcined magnesite sourcecan also be ground to better cause dissolution in the mixture.

In an embodiment, the systemcan also include an air sourceconnected to the interiorof the crystallizerand operably connected to the control systemsuch that the control systemcan create a predetermined velocity gradient based on the pressure and flow rate of the air supplied into the interiorof the crystallizerby the air source. Here, the air sourceis a pump and includes a pressure regulatorand rotameter to assist in maintaining pressure and flow rate into the interior.

Additionally, the crystallizercan further contain one or more screens to control foaming. Here, the screen inside the crystallizerhas a small mesh number and the screen in the inlethas a larger mesh number to help with reduce foaming. The calcined magnesite source, water-soluble magnesium mineral source, and wastewater sourcecan all be completely held inside the crystallizerfor batch operation, or can be continuously flowed through the crystallizerand periodically remove struvite from the flowing mixture.

The present invention takes advantage of the phenomenon that when there is a significant increase in the ionic activity product of phosphate (PO), magnesium (Mg), and ammonium (NH), and the wastewater becomes supersaturated with them, struvite crystallization will occur spontaneously. The recovery process of struvite typically follows two steps: crystal nucleation and crystal growth. Nucleation is crystal birth in which crystal embryos are formed and crystal growth is the development of crystals after crystal embryos have been born and it determines the final size of the crystals. The time it takes to form struvite nuclei is known as the induction time.

A main factor for struvite precipitation is the supersaturation of constituent ions in wastewater. Extended retention time of the crystal suspension under supersaturated conditions leads to an increase in struvite crystal size. Therefore, longer retention times in days are required to agglomerate the crystals to form fertilizer sizes of granules within the reactors.

In practical applications, slightly higher Mgto POratios have proved to be more efficient for phosphorus recovery as struvite. A Mgto POratio of 1.2 proved to be optimum for struvite recovery from anaerobically digested swine wastewater, anaerobic sludge supernatant and urine.

The pH for effective recovery of phosphorus as struvite is generally in the range of 8.5-9.5. In this pH range, the solubility of struvite is lowest, nitrogen is present mainly in the form of NH, and phosphate exists as HPO, making conditions suitable for struvite precipitation. Therefore, in most cases of struvite production, chemicals in the form of alkalis are added to increase pH for recovering struvite from wastewater.

The present invention can be used to recover struvite from different waste streams, such as anaerobic digester liquors, livestock and farm wastes, human urine, and landfill leachate, among others.

As the present invention teaches, careful control of solution pH, molar ratios of constituent ions, and residence time is essential in the design of struvite reactors to achieve a higher efficiency in struvite recovery.

the most used magnesium sources for struvite production are MgCl, MgSO, and Mg(OH)both at laboratory scale studies and commercially available struvite recovery systems. MgSOand MgClresult in high salt concentrations and electrical conductivity in the resulting effluents whereas the lower solubility and thus longer reaction time renders Mg(OH)less feasible for struvite production. The use of these salts is complemented by the utilization of alkali to control pH for the recovery process. These pure chemicals are expensive and increase the overall cost of production. Thus, alternative magnesium sources can improve the process economics and make struvite recovery profitable. Certain low-cost alternatives to chemical magnesium sources have been utilized including desalination bittern, seawater, thermally decomposed magnesite, acid-treated brucite, and milled serpentine, but their availability is limited to local regions, and their use may elevate the ionic strength of wastewater with the introduction of interfering ions.

With respect to the calcined magnesite source, Magnesite (MgCO) is a mineral present abundantly in nature. This mineral is the predominant source to produce magnesia (MgO) worldwide and as compared to chemical magnesium reagents, its cost is only about 1/10. Magnesite can be composed of as much as 98% of MgCOmaking it a rich source of magnesium. However, it is sparingly soluble in aqueous solutions and its solubility in water is 0.01 g/L at 25° C. Therefore, to use it for struvite precipitation, it must be converted to reactive magnesium oxide by applying certain pretreatment such as acid extraction. However, these methods to increase its reactivity are chemical intensive.

Another way of increasing the reactivity of magnesite is calcining it after particle size reduction (Al-Mallahi et al., 2020). Calcination of magnesite mineral is carried out at high temperatures to decompose it to magnesium oxide (MgO) and carbon dioxide:

The effects of calcination temperature and duration on the dissolution of calcined magnesite in rare-earth processing wastewater, synthetic phosphate-depleted human urine, acidic electroplating wastewater, and liquid digestate, respectively, are known. Another benefit of the use of calcined magnesite is the production of hydroxyl ions upon dissolution in water, leading to an elevation in the pH of wastewater.

Thus, calcined magnesite is versatile in providing both Mgand OH ions to the wastewater simultaneously fulfilling the need to supplement magnesium and elevate pH for struvite recovery. The present invention has optimized the calcination process at light burnt temperature of 800° C. for 30 minutes to produce calcined magnesite for effective struvite formation.

Synthetic Epsom salt with the formula MgSO7HO is a highly soluble salt and its solubility in water is 732 g/L at room temperature and pH of magnesium sulfate solution is slightly acidic. Owing to its high solubility, it has been successfully used as a magnesium source for struvite recovery. However, when synthetic Epsom salt is used to supplement magnesium, pH is increased using NaOH thus rendering the process expensive and struvite unsuitable for organic production. Conversely, Epsom salt derived from Epsomite minerals following good manufacturing practice, such as the OMRI-listed Magriculture® Epsom salt, can be directly applied to provide magnesium supplementation. Since its application is allowed in organic farming, struvite recovered from it can potentially be suitable for use in organic farming.